Since about 1960, the so-called common depth point ("CDP") or common reflection point seismic technique has been almost universally used to improve the quality of seismic records so as to make the records more susceptible to interpretation. The CDP technique involves carefully spacing seismic sources and seismic detectors, and combining individual seismic record traces so that reflection events from common reflection points (or locations) on subsurface strata are additively combined to reinforce each other, and unwanted noise events are canceled.
In performing CDP operations, seismic traces, each representing an acoustic signal received at a seismic receiver after having propagated away from a seismic source, are gathered into sets of traces ("CDP gathers") in which each trace of the set represents reflections from a common subsurface point. The primary reflection signals along the traces in each CDP gather all fall along a generally hyperbolic curve known as the normal moveout curve. Typically, the seismic traces in each CDP gather are processed to compensate for the time differentials in the occurrence of the primary reflection signals caused by this normal moveout. Such compensation aligns all the primary reflection signals at the same point on the time axis of the seismic traces. Conventionally, the seismic traces are then stacked to form a composite trace for each common reflection point, such stacking again enhancing the primary reflection signals relative to multiples and noise. These composite traces are then conventionally displayed in side-by-side relationship in the form of a seismic section. Such a seismic section indicates continuous primary seismic reflections and is a useful tool for the geophysicist in determining the acoustic velocity characteristics of the subterranean formation associated with such continuous reflections.
It is well known that the variation of the reflection amplitude of seismic traces with angle of incidence with respect to a reflecting subterranean interface, or with "offset distance" (i.e., distance between the source and the receiver associated with a particular trace), may be interpreted as an indicator of the presence or absence of gas in association with a reflecting subterranean interface.
For example, each of U.S. Pat. No. 4,316,267 issued Feb. 16, 1982 to Ostrander and U.S. Pat. No. 4,316,268 issued Feb. 16, 1982 to Ostrander discloses a method of preparing and interpreting a display of seismic data for the purpose of distinguishing gas related "bright spots" (high-amplitude anomalies in seismic traces) from other bright spots. In the most basic form of practicing this method, a single stacked trace (representing a stack of individual seismic traces in a CDP gather of traces) passing through the bright spot of interest on a seismic section is selected for analysis. This stacked trace is separated into the individual traces from which it was originally composed. These individual traces are then displayed side-by-side in order of progressively changing source-receiver offset distance. The resulting display permits an interpreter to examine the reflectivity of the reflecting interface of interest as a function of source-receiver offset distance. The Ostrander patents teach that, for reflections from the upper boundary of a gas reservoir, the amplitude of the reflection usually varies significantly as a function of offset, while for reflections from other interfaces the amplitude is in most instances substantially independent of offset.
It has long been known that the amplitude of the reflections of a seismic signal from the layered structure of a subterranean hydrocarbon reservoir are frequency dependent. See, for example, N. A. Haskell, "The Dispersion of Surface Waves on Multilayered Media," Bulletin of the Seismological Society of America, 1953, Vol. 43, No. 1, pp 17-34.
The related phenomenon of the frequency dependence of amplitude attenuation of seismic waves which have propagated through a subterraneam formation is employed in the geophysical exploration technique disclosed in U.S. Pat. No. 3,292,143, issued Dec. 13, 1966 to Russell. Russell discloses filtering a seismic trace through two band-pass filters, and then producing a signal whose amplitude at any time is the ratio of the amplitudes of the two signals produced at the output of the filters.
However, Russell neither discloses nor suggests band-pass filtering the traces comprising a CDP gather. Nor does Russell disclose or suggest producing a display of seismic data in which band-pass filtered seismic traces associated with a common reflection point are displayed in order of progressively changing offset values or frequency content, for the purpose of facilitating direct hydrocarbon identification or for any other purpose.
Nor do the Ostrander patents discussed above disclose any method for examining amplitude variations with frequency, as an indicator of the presence of hydrocarbons in a subterranean earth formation or for any other purpose. Prior to the present invention, frequency-dependent amplitude variations have not been used as a direct hydrocarbon indicator. The present invention is a technique for preparing a display of seismic data for the purpose of facilitating the use of frequency-dependent amplitude variations, as well as the use of offset-dependent amplitude variations, as a direct indicator of the presence of hydrocarbons in a subterranean earth formation.
The present invention facilitates an improved direct hydrocarbon identification technique by providing a new type of seismic data display. The display of the invention permits convenient examination of the reflectivity of a subterranean interface of interest as a function of both frequency and source-receiver offset. Until the present invention, seismic data has not been displayed in a manner permitting convenient examination of both the source-receiver offset dependence and frequency dependence of reflectivity. The inventive display has the additional advantage of reducing the effect of noise on data interpretation, since the dominant contribution of noise is likely to be in a different frequency range than are the primary reflections of interest. Also, the inventive display reduces the effect of attenuation on data interpretation by permitting examination of offset-dependent data over a selected narrow frequency range.